Method of coating process aluminum and its alloys



METHOD OF COATING PROCESS ALUMINUM. AND ITS ALLOYS Morris A. Steinberg,University Heights, Ohio, assignor to Horizons Incorporated, Princeton,N. J., a corporation, of New Jersey No Drawing. Application April 15,1955 Serial No. 501,709

3 Claims. (Cl. 148-6.11)

This invention relates to the coating of aluminum and alloys which arecomposed principally of aluminum. More particularly, it relates to aprocess in which the aluminum is coated with metals which are belowaluminum in the electromotive series.

In accordance with my invention, objects formed of aluminum or aluminumalloys are coated either by simple immersion in a reactive compositionor by such immersion followed by dipping into a molten bath of the metalor alloy which is to serve as the coating.

The production of adherent metallic coatings on aluminum and alloys ofaluminum has been extensively investigated. Workers in this art havelong recognized that the problem is complicated by the superficial oxidefilm that invariably is formed on any freshly exposed aluminum surfaceby reaction with the ordinary atmosphere. Failure to effect the removalof all of this oxide film leads to coatings which flake off because ofpoor adherence to the oxide. Such coatings, therefore, fail to providethe desired benefit for which they were applied, for instance insoldering or joining the aluminum. I

It has been generally proposed to remove the superficial oxide filmeither by mechanical means or by chemical action as a conditionprecedent to the application of any coating metal. One commonly adoptedexpedient has been to fioat a molten flux layer upon a molten bath of acoating metal and to dip the aluminum or aluminum alloy object into theflux and thence, after the surface has been chemically cleaned, into themolten metal bath.

I have now discovered that the above procedure may be greatly simplifiedand that satisfactory, tightly adherent coatings may be produced on thealuminum objects without the necessity of a molten metal coating bath.In

accordance with one procedure, I provide a molten flux bath in which oneof the essential constituents comprises a combination of chlorides whichwill dissolve the aluminum oxide film, and one or more metal chloridesof a metal with which the freshly exposed aluminum base metal willreact, and by simple displacement will decompose the metal chloride andcause the metal to deposit thereon as a metallic coating.

In another embodiment of my invention I provide flux consisting of theabove ingredients and disposed over a molten bath of the metal to bedeposited as a coating on the aluminum or aluminum alloy surface. Inthis modification of my invention I have found it preferable to includein the flux the chloride of the metal or metals forming the molten bathupon which my reaction flux floats.

One object of my invention is to provide a composition which willeffectively remove the oxide layer from aluminum objects at temperaturesbelow 300 C. and at the same time will effectively deposit a-metal onthe freshly cleaned aluminum or aluminum alloy surface.

Another object of my invention is to providea simple and inexpensivemethod for coating aluminum with a uniform, tightly adherent metalliccoating. A further United States PatentO 2.53am Patented May 6, 1958 2object of my invention is to provide a composition and method which areparticularly effective for removing the oxide film from the surface ofaluminum or aluminum below 300 C. and a second component which comprisesthe chloride of at least one metal below aluminum in the electromotiveseries is prepared in the following manner.

The first component is prepared by mixing aluminum I chloride and one ormore alkali metal chlorides of the group consisting of lithium chloride,sodium chloride, and potassium chloride and heating the mixture gentlyto a temperature below 300 C. at which the constituents form a clearmelt. The melt may be solidified and comminuted and stored for futureuse or it may have added to it the second essential constituent of my.composition.

Chlorides which have been found suitable as the second constituent arethe following:

AgCl HgCl AuCl InCl AuCl MnCl CdCl NiCl CoCl v PbCl 8%? fi s CuCl SHCIZFeCl TlCl GaCl ZHCI When it is desired to coat an aluminum object by thefirst procedure, simple immersion in the reaction flux, a mixture of thealuminum chloride-alkali metal chloride component with the coating metalchloride or chlorides is melted in a suitable vessel and stirred toobtain a homogeneous mixture. Since many of these mixtures will melt attemperatures as low as 200 C., inexpensive glass vessels or glass-linedvessels may be employedwith substantial economic advantages. Once thecomposition has been melted and homogenized,it is merely necessary toimmerse the aluminum or aluminum alloy object into the flux and to moveit gently back and forth therein'in order to insure an intimatecontactbetween the'fiux and all surface portions of the object to be coated.

The relative proportions of aluminum chloride, alkali metal chloride andcoating metal chlorideor'chlorides are selected so that the homogenizedcomposition will have a relatively low melting point preferably belowabout 200 C. before it is used as well as after it is used. To produce acomposition with the desired low melting point, I have found that theproportion of aluminum chloride in the aluminum chloride-alkali metalchloride component of the mixture should be between 52 mol percent and82 mol percent and the proportion of alkali metal chloride in thealuminum chloride-alkali metal chloride component should be between 48mol percent and 18 mol percent; and the amount of coating metal chlorideshould constitute less than about 50% by weight of the mixture andshould constitute at least a sufficientamount to de-.

ride-alkali metal chloride component that the'composition tends to loseto a significant extent the ability to dissolve the aluminum oxide filmand thatwith suchamounts of aluminum chloride the composition tends tobecome unstable and to lose aluminumchloride by volatil-ization Once theoxide film has been removed from the aluminum or aluminum alloy objectto be coated, the exposed metal reacts with the coating metal chlorideor chlorides present in the molten flux as follows: i

where M 01, is a metal chloride chosen from the above mentioned metalchlorides and x and y are integers.

The following examples will further illustrate the preferred manner ofpracticing this aspect of my invention:

Eitample I Example II A clear melt was prepared by heating a chargecomprising 60 parts by weight of aluminum chloride covered by 20 partsby weight of sodium chloride and 20 parts by weight of stannous chlorideto a temperature of about 300 C. A piece of aluminum was immersed in themelt for 2 minutes after which itvwas withdrawn. A bright,

, uniform tin platewas obtained on thealuminum free from spots orstreaksr I The second aspect of my invention involves the use of amolten metal bath disposed beneath a supernatant molten flux layer. Theprocedure is substantially similar to that previously outlined forpreparing the flux and the fol lowing examples will serve to illustratethe preferred practice of this aspect'of my invention:

1 Eatample III A flux composition was prepared by melting 60 parts byweight of aluminum chloride, 20 parts by weight of sodium chloride and20 parts by weight of stannous chloride in a glass beaker covered with awatch glass by heating toabout 300 6;, at whichtemperaturea clear meltformed -Into this melt there was added about 2000 grams of metallietin.whichfon continued heating, was readily melted and form'e'dga poolbeneath the molten flux. A piece of aluminum was immersed in the fluxand then droppedintothe moltentin, Upon withdrawal from the bath it wasfound to be covered completely with a uniform, tightlyadherent layer oftin, free from streaks or spots. 1 t t Example I V t A flux compositioncomprised of 60 parts by weight of aluminum chloride, 20, parts byweight of sodium chloride, 10 parts byiweight ofc'admium chloride and 10 parts by weight of zinc'chloridelwas charged into a glass beaker andmelted as in Example, IIIL Into. this melt there was added about 2500grams of a cadmiumzinc; alloy which was readily meltedtherein. ,A pieceof aluminum was immersed first in the flux and thenin the molten metaland when withdrawn was found to be covered with a layer of acadmium-zinc alloy This combination of baseand coating is particularlyadvantageous since the. cadmium zinc does not form an electrolyticcouple with the alumi num base and henceis particularly resistant;tocertain types of corrosion. i i Y discovered am thod rd po ing metallc "coatings on i s o a umi m or al oys. of a .uminu,ni e the y Fromthe'above examples it will be apparent-that I have, i

simple immersion in a reactive flux containing one or more chlorideswhich react with the aluminum and a combination of ingredients whichdissolve the aluminum oxide film from the object to be coated or byimmersion in such a flux followed by immersion in a molten metal ormetal alloy bath.

While there are enumerated a large number of chlo-' rides which havebeen found to be satisfactory for the production of coatings, it will beunderstood that for reasons of economy, the chlorides of lead, tin,cadmium and zinc, and combinations of these chlorides are preferred inthe practice of my invention.

Furthermore, although the flux compositions described will generally bemolten at temperatures below about 300 0., they may be employed incooperation with molten metal or alloy compositions which melt atappreciably higher temperatures. Thus, the flux bath of Example 1 hasbeen employed as the supernatant flux on a pool of molten zinc (M. P.420 C.) without excessive fuming or breakdown of the flux.

Another factor which it is necessary to consider in the practice of mymethod is the choice of appropriate reactive chlorides to be added tothe aluminum chloride-alkali metalchloride basic flux composition.Obviously, chlorides shouldbe chosen which do not react with theunderlying pool of molten metal, since this would lead to waste of bothchloride and molten bath material. For this reason,

. it has been found particularly useful to add to the flux,

the chloride of the coating metal over which the fiux is floated. Ofcourse when the flux is employed without any molten pool, any of thechlorides above listed may be used.

By filming first a coating of a metal on the aluminum substrate bydecomposition of a chloride constituent of the fiuxand then dipping anarticle into a diiierent metal, it is possible to provide coatings ofimproved adherence to the base metal, particularly where theintermediate metal is one which bonds more readily to the substrate thanthe outermost metal.

The term aluminum in the following claims is intended to cover the metalitself as well as alloys which are composed principally of aluminum,particularly alloys with upwards of of aluminum.

I claim:

1. The method of coating aluminum and aluminumbase alloys with a coatingmetal which is more electronegative than aluminum, which comprisesestablishing a molten body of the coating metal, forming on at least aportion of the surface of said molten body of coating metal a fused,homogeneous salt composition comprising a major amount of an aluminumchloride-alkali metal chloride constituent consisting essentially ofbetween about 52 mol percent and 82 mol percent of aluminum chloride andbetween about 18 mol percent and 48 mol percent of at least one alkalimetal chloride from the group consisting of sodium chloride, potassiumchloride and lithium chloride, and between about 1 percent and 25percent by weight of the aluminum chloride-alkali metal chlorideconstituent of a chloride ofat least one metal from the group consistingof zinc chloride, stannous chloride, lead chloride and cadmium chloride,passing the article to be coated downwardly through the supernatantfused salt composition and into the underlying molten body of coatingmetal, whereby the aluminum oxide film on the article is removed and acoating of the metal of the metal chloride of said group of metalchlorides is formed on the article, as the article passes through thesupernatant fused salt composition and before it is immersed into themolten body of the coating metal, and thereafter withdrawing the articlefrom the molten body of said coating metal with a coating-of the coatingmetal thereon.

2. The method of coating aluminum and aluminumbase alloys with a coatingmetal which is more electro-- negative than aluminum, which comprisesestablishing a molten body of the coating metal, forming on at least aportion of the surface of said molten body of coating metal a fusedhomogeneous salt composition consisting essentially of between about 52mol percent and 82 mol percent of aluminum chloride and between about 18mol percent and 48 mol percent of at least one alkali metal chloridefrom the group consisting of sodium chloride, potassium chloride andlithium chloride, and between about 1 percent and 25 percent by weightof a chloride of at least one metal more electIonegative than aluminum,passing the article to be coated downwardly through the supernatantfused salt composition and into the underlying molten body of coatingmetal, whereby the aluminum oxide film on the article is removedtherefrom and a coating of the metal of the metal chloride moreelectronegative than aluminum is formed on the article as the articlepasses through the supernatant fused salt composition and before it isimmersed into the molten body of the coating metal, and thereafterwithdrawing the article from the molten body of the coating metal with acoating of the coating metal thereon.

3. The method of coating aluminum and aluminumbase alloys with a coatingmetal which is more electronegative than aluminum, which comprisesestablishing a molten body of a fused, homogeneous salt composition,consisting essentially of a major amount of aluminum chloride and aminor amount of at least one alkali metal said coating metal, immersingthe aluminum article in the fused salt bath, whereby the oxide film onthe articleis removed by the molten salt and a coating of the metal ofthe metal chloride more electronegative than aluminum is formed on thearticle, thereafter further coating the freshly coated article byimmersing it into the body of molten metal, and withdrawing the articlefrom the body of molten metal with a final coating thereon.

References Cited in the file of this patent UNITED STATES PATENTS1,293,823 Luckey Feb. 11, 1919 1,534,105 Gann Apr. 21, 1925 1,550,280Post .Aug. 18, 1925 2,299,166 Miller Oct. 20, 1942 2,330,904 Miller Oct.5, 1943 2,443,169 Shanklin June 8, 1948 2,480,498 Miller Aug. 30, 19492,481,053 Wassermann Sept. 6, 1949 2,596,466 Bowden May 13, 19522,674,790 Edson et a1. Apr. 13, 1954 2,723,929

Wagner et a1 Nov. 15, 1955

1. THE METHOD OF COATING ALUMINUM AND ALUMINUMBASE, ALLOYS WITH ACOATING METAL WHICH IS MORE ELECTRONEGATIVE THAN ALUMINUM, WHICHCOMPRISES ESTABLISHING A MOLTEN BODY OF THE COATING METAL, FORMING ON ATLEAST A PORTION OF THE SURFACE OF SAID MOLTEN BODY OF COATING METAL AFUSED, HOMOGENEOUS SALT COMPOSITION COMPRISING A MAJOR AMOUNT OF ANALUMINUM CHLORIDE-ALKALI METAL CHLORIDE CONSTITUENT CONSISTINGESSENTIALLY OF BETWEEN ABOUT 52 MOL PERCENT AND 82 MOL PERCENT OFALUMINUM CHLORIDE AND BETWEEN ABOUT 18 MOL PERCENT AND 48 MOL PERCENT OFAT LEAST ONE ALKALI METAL CHLORIDE FROM THE GROUP CONSISTING OF SODIUMCHLORIDE, POTASSIUM CHLORIDE AND LITHIUM CHLORIDE, AND BETWEEN ABOUT 1PERCENT AND 25 PERCENT BY WEIGHT OF THE ALUMINUM CHLORIDE-ALKALI METALCHLORIDE CONSTITUENT OF A CHLORIDE OF AT LEAST ONE METAL FROM THE GROUPCONSISTING OF ZINC CHLORIDE, STANNOUS CHLORIDE, LEAD CHLORIDE ANDCADMIUM CHLORIDE, PASSING THE ARTICLE TO BE COATED DOWNWARDLY THROUGHTHE SUPERNATANT FUSED SALT COMPOSITION AND INTO THE UNDERLYING MOLTENBODY OF COATING METAL, WHEREBY THE ALUMINUM OXIDE FILM ON THE ARTICLE ISREMOVED AND A COATING OF THE METAL OF THE METAL CHLORIDE OF SAID GROUPOF METAL CHLORIDES IS FORMED ON THE ARTICLE, AS THE ARTICLE PASSESTHROUGH THE SUPERNATANT FUSED SALT COMPOSITION AND BEFORE IT IS IMMERSEDINTO THE MOLTEN BODY OF THE COATING METAL, AND THEREAFTER WITHDRAWINGTHE ARTICLE FROM THE MOLTEN BODY OF SAID COATING METAL WITH A COATING OFTHE COATING METAL THEREON.